Traditional 911 call distribution arrangements servicing complex metropolitan areas utilize a centralized switch to route police and fire emergency calls, known generally as 9-1-1 calls, to a correct Public Safety Answering Point (PSAP). Additional equipment is required at the PSAP to distribute calls to answering positions. Enhanced 911 (E911) service, an improvement over the traditional 911 service, consists of three major features: (1) selective routing of the 911 calls, (2) Automatic Number Identification (ANI), and (3) Automatic Location Identification (ALI). Selective routing, implemented through a switching system, routes a call to a correct PSAP, which is typically the PSAP designated to serve a calling party's calling area. In general such a routing is achieved by an end office by switching the incoming 911 call to a tandem switching system serving the PSAP via a trunk connection, and delivering the calling party's identification information as well as any location identification information in an information packet (in the case of a digital switching system) or by out-pulsing digits (when using an in-band switching protocol) to a destination switching system located at the designated PSAP. In cases where the location information is not delivered, the ANI is used as a key to look up the location information in a database.
Until recently, an incumbent local exchange carrier (ILEC) such as a Regional Bell Operating Company (RBOC) handled most of the 911 calls. A 911 call that typically originates within a service area of an ILEC end office switching system is routed to an E911 tandem switching system. The 911 emergency telephone services are also known as "lifeline" services, since they are a first recourse for a person in distress. In order to maintain an error-free operation of these lifeline services, telephone companies generally configured their networks to provide a fault-tolerant operation and very high availability. In general this is achieved by providing a "full connectivity" among the switches that route a call from a calling party's telephone number to a PSAP. In a "full connectivity" switching architecture, diverse dedicated trunks from every end office switching system to an E911 tandem switch are provided for each E911 service area. An example of this architecture is shown in FIG. 1 (PRIOR ART).
Referring now to FIG. 1 (PRIOR ART), a traditional "full connectivity" architecture is shown. One of a pair of E911 tandem switches 102 serving a PSAP 103 is connected by primary links 104a, 104b, 104c and also by secondary links 106a, 106b, 106c to each of three corresponding end office-type switches 108a, 108b, 108c, respectively. In FIG. 1, each of the end-office switches serve as gateways to a E911 tandem switch, and thus serve as gateway switches. Gateway switches 108a, 108c, 108c connect subscribers (i.e., customers) 110, 112 to the E911 tandem switch 102 through the primary and secondary links, and also connect the subscribers to a switch network 114 for connection to long distance and other services.
The gateway switches 108a, 108b, 108c are typically end office switches such as a Lucent 5ESS switching system, a Nortel DMS 100/200 switching system, or a Lucent 4ESS toll tandem. It is understood in FIG. 1 that the number of gateway switches is not limited to three and that multiple subscribers are serviced by any one gateway switch. It is also understood that gateway switch 108b also serves subscribers, though none are shown in the drawing. It is also understood that the links 104a-c and 106-c are typically trunk groups capable of handling multiple calls simultaneously.
A 9-1-1 call from subscriber 110 is placed via corresponding gateway switch 108a. When such a call is received by gateway switch 108a, that switch attempts to connect the subscriber to the E911 tandem switch via the primary link 104a. If the primary link 104a is unavailable due to failure or overload, the 9-1-1 call is routed via the secondary link 106a. Redundant primary and secondary links from each gateway switch help ensure that a 9-1-1 call goes through. Full connectivity in this context means that each gateway switch is provided with primary and secondary links to the E911 tandem switch.
As seen in FIG. 1, the prior art includes situations in which, for additional reliability, E911 tandems are deployed in mated pairs. This ensures that even if one E911 tandem ("E911-A") is down, its teamed E911 tandem ("E911-B") will serve as a backup. This ensures that the emergency call will go from the gateway through an E911 tandem and on to the proper PSAP, even in the event of a catastrophic loss of one of the E911 tandems. When mated pairs of E911 tandems are used, full connectivity comes in two forms: one in which each gateway is provided with a single link to each member of the E911 tandem pair, and a second in which each gateway is provided with primary and secondary links to each member of the E911 tandem pair.
The prior art also includes situations in which an end office must maintain multiple pairs of links to the E911 tandem, or to a multiplicity of 911 tandems, with each pair of links dedicated to 911 calls originating in a designated part of the end office serving area.
Such arrangements of providing a full connectivity architecture to ensure a fault-tolerant operation of the emergency lifeline services is feasibly for an ILEC switching system because, typically an ILEC end office switching system's service area is a small geographical area, and an ILEC typically serves each particular geographic location from a single end office.
With the passage of the Telecommunications Act of 1996, there arose an opportunity for competitive local exchange carriers (CLECs) such as AT&T to enter the local exchange market and start providing services similar to those offered by incumbent LECs (ILECs) to their residential and commercial customers. While the traditional ILEC end office service area is confined to a small geographical area such as a borough of a city, a CLEC may provide service to a very large geographical area from a single end office. In additional configurations, a CLEC may serve a single geographical area from many end offices. In such a scenario, it would not be feasible for the CLEC to configure its E911 switching architecture in a similar manner to that of an ILEC, i.e., according to a "full connectivity" architecture. This is because there will be an excessive amount of trunks connecting each end office with the E911 tandem 102, or a multiplicity of such E911 tandems when the CLEC end office serves a large geographic area, which could be prohibitively expensive.
Therefore, there is a need for a system and method of interconnecting end office switching systems owned and/or operated by a single entity with E911 tandem switches in a manner that requires fewer trunks than in a "full connectivity" architecture while still providing a fault-tolerant and highly available E911 service.